In recent years, a technique to form optical functional elements on a silicon substrate cheaply by utilising silicon electronic circuit technology capable of large-scale integration has become a focus of attention. Amid improvements in the performance of information processing machinery and equipment, ranging from high-performance servers and supercomputers all the way down to personal computers, there is concern over the capacity bottleneck of communication between chips and boards. As a technique to solve this problem of a communication capacity bottleneck, hopes are pinned on the realization of compact low-loss optical elements on silicon substrates using silicon wire waveguides.
To realize a large-scale optical element on a silicon substrate, there is a need for a light source that provides the optical element with a signal light. However, as an indirect gap semiconductor, silicon is not suited for the implementation of a high-efficiency light source. As a technique to solve the problem of a communication capacity bottleneck, the realization of a hybrid-integrated laser source comprising a high luminous-efficiency light source based on InP, GaAs or some other semiconductor compound and optical elements provided on a silicon substrate is regarded as promising.
For instance, an optical element comprising two or more ring resonators that is designed to strictly control the oscillation wavelength by taking advantage of the Vernier effect of the resonant wavelengths of the ring resonators has been proposed. An optical element featuring a wavelength filter configured by cascade-connecting three ring resonators and a ring resonator-type light modulator has also been proposed.    Patent Document 1 Japanese Laid-open Patent Publication No. 2006-245344    Patent Document 2 Japanese Laid-open Patent Publication No. 2010-027664